H - Revised Integrated Water Management Plan by Cardno (inc MUSIC file) (13.09.19)

                                                                                  Integrated Water Management Plan
                                                                          Noosa Marketplace : 2-20 HOFMANN DRIVE




2         Request for Information

2.1       Background
On the 28th June 2019, Noosa Council issued an information request under The Noosa Plan requiring further
information to satisfactory assess the proposal.

2.2       Information Required
Issue – Stormwater Quality
The litter baskets and rainwater tanks are supported, however, the proprietary devices are not. The rainwater
tanks have a high reuse demand which presumably is based on irrigating all of the landscaped areas of the
site and possibly the retained covenant vegetation. This may not be appropriate and further details should be
provided on their assumptions.
Information Required
Provide further information and/or a revised design and model as necessary to address the following items:
a. Provide justification for the assumed annual reuse demands for each rainwater tank and provide estimates
of the source reliability (i.e. how much of the demand will need to be met by mains back-up supply). In general,
use of irrigation water for landscaping is not encouraged and low-water-usage native plants should be used in
accordance with Specific Outcome O3 of the Water Sensitive Design Code. Water reuse should be focused
on areas of water demand which cannot be addressed through changes in design/planting – such as for toilet
flushing.
b. The use of proprietary underground devices for nutrient removal is in conflict with O7 of the Water Sensitive
Design Code. Provide a revised stormwater treatment train and revised landscape design for the car parking
areas which utilizes vegetated treatment strategies to achieve multiple benefits.
c. Provide details of the assumed removal rates for the litter baskets and/or the MUSIC model for review.
Response
Section 6 of the IWMP has been updated to cover a). b) and c) above with comments below.
MUSIC model water reuse has been revised to only use rainwater tanks for non-potable uses with water reuse
rates calculated from sewer demands assuming 600 L/day/ET.
Proprietary devices have been removed from the development and treatment provided with a proposed
combination of trash baskets, bio-retention tree pits and/or filter strips. Table 6-5 outlines the typical bio-
retention basin parameters. These treatment nodes have been modelled as two representative areas in the
MUSIC model.
Litter basket removal rates were sourced from the supplier. These are shown in Table 6-6
Issue – Hydrology
The pre-development hydrologic model uses relatively low initial loss values, particularly for the rarer AEP
events. The IWMP acknowledges that the values differ from those provided by ARR Datahub but states that
the values adopted are conservative. While the values (low initial loss) would be conservative if the purpose
was to undertake flood mapping or estimate design flood levels, this approach is not considered conservative
for the stated purpose of determining hydrologic and flood impacts of the proposal. Low initial loss values
increase pre-development peak flows and peak flood levels which in turn can underestimate hydrologic and
flood impacts and required flood detention storage volumes. This issue is highlighted by the predicted RAFTS
flows being up to 63% greater than the rational method estimates for the pre-developed case.
The subject site is subject to local overland flowpath flooding from external catchments located to the North
and West of the subject site which are of the order of 7Ha in size. These catchments are conveyed through
the site as overland flow in a South-Easterly direction and ultimately discharge under Hofmann Drive to an
existing conservation area.
The proposal has the potential to worsen flooding conditions upstream of the site by blocking/modifying this
overland flowpath and has the potential to worsen downstream flooding by increasing peak flowrates
downstream of the site by increased runoff peaks and volume (due to increased imperviousness of the site).
Information Required


V190428 | 10 September 2019 | Commercial in Confidence                                                          4